Strain-Induced Phase Transformation Modeling of QP980 Steel and Its Application to Complex Loading Paths

Abstract

Quenching and partitioning (QP) steel has attracted much focus due to the effect of phase transformation induced plasticity (TRIP). However, the TRIP behavior makes it difficult to accurately predict the strain and stress distribution as well as the phase transformation behavior of QP steel. The scanning electron microscope (SEM) images of QP980 microstructure were obtained by scanned electron microscope in this study, which was characterized by the combination of lath martensite, polygonal ferrite and retained austenite. Volume fraction evolution of retained austenite with equivalent plastic strain (EPS) of uniaxial tension was obtained by electron back scatter diffraction. The phase transformation kinetics equations of QP980 were deduced based on the phase transformation model proposed by Olson and Cohen (simplified as O-C theory), considering the effects of strain rate, deformation temperature and stress state. A constitutive model on the dependence of the phase transformation was proposed to reveal the relation between metallographic characteristics and mechanical performance of QP980 steel during deformation. Users’ subroutine VUMAT in ABAQUS/Explicit was implemented to describe the volume fraction of retained austenite (VFRA) under different stress states. The established phase transformation and constitutive model were applied to three kinds of complex path loading tests. The variation of retained austenite under complex strain paths was obtained and compared with the experimental results.